How candidates actually lose marks

A worked solution tells you the right answer. It does not tell you why the answer you chose looked right — which is the only question worth asking after getting one wrong.

We went through every wrong option in the previous-year papers and worked out the specific mistake that produces it. 286 of them so far, across 13 recurring kinds of error. They are ranked below by how often each one actually appears — not by how important it sounds.

1. 01

   Execution slip128 wrong options · Part A, B, C

   The idea was right. The computation was not.

   See examples
2. 02

   Boundary and endpoint53 wrong options · Part A, B, C

   The statement turns at the edge of the interval, the domain, or the parameter range.

   See examples
3. 03

   Hypothesis dropped31 wrong options · Part A, B, C

   A theorem applied without checking one of its preconditions.

   See examples
4. 04

   Standard counterexample31 wrong options · Part A, B, C

   There is a canonical object that settles this. Recognising it is the whole question.

   See examples
5. 05

   Converse assumed13 wrong options · Part A, B, C

   The theorem runs one way. You used it in the other.

   See examples
6. 06

   Base field or ring10 wrong options · Part B, C

   The answer changes with the field or ring you are working over.

   See examples
7. 07

   Invariants don't determine8 wrong options · Part B, C

   Two objects sharing an invariant were treated as the same object.

   See examples
8. 08

   Pointwise vs uniform4 wrong options · Part C

   The mode of convergence — or of continuity — was the wrong one.

   See examples
9. 09

   Dependence misread2 wrong options · Part B

   Independence, pairwise vs mutual, or correlation vs dependence.

   See examples
10. 10

    Moments and tails2 wrong options · Part C

    A moment assumed to exist, or tail behaviour assumed to be tame.

    See examples
11. 11

    Property not inherited2 wrong options · Part B

    A property assumed to pass to subobjects, quotients, or through a chain. It does not.

    See examples
12. 12

    Existence vs uniqueness1 wrong option · Part C

    A theorem giving one was read as giving both.

    See examples
13. 13

    Limit assumed to exist1 wrong option · Part B

    Reasoning with a limit before establishing there is one. limsup is not lim.

    See examples